Well, I gave you my opinion.
But this black oil could also be due to your modifications. A rich fuel mixture is NOT going to pollute your oil UNLESS your getting an a$$ load of blow by. Which would give you one heck of a smokey, rich exhaust system.
My estimation is that these modifications:26" XTRs, Jet Kit, HMF Utility Exhaust, K&N Air Filter w/outerwears. Are what is causing your oil to turn black so quickly. Your engine is working harder and is producing more power. This wears on your clutch more as well as creates more heat and is causing your oil to age faster then the normal service intervals. It should have nothing to do with your fuel mixture. Adjust your service intervals to meet the demands of your modified machine.
Next time you change your oil, make sure you ride the bike until it is HOT. Then drain the oil, and change the filter. You could even do an engine flush if you're feeling ambitious.
If the oil turns black again after 50 miles, and this bugs you, have a Honda certified mechanic service the quad.
Mike